am/is/are+being+done这个结构中为什么要being+done?

am/is/are+being+done这个结构中为什么要being+done?
am/is/are+being+done这个结构中为什么要being+done?

am/is/are+being+done这个结构中为什么要being+done?
am/is/are+being+done 是现在进行时的被动词语态.
The trees are being cut nowadays .
加油!不明白再问!